The dual-tone multifrequency is a main method for transmitting numbers through tones in a voice channel in a voice call, and it is invented by Bell Laboratories. The dual-tone multifrequency represents one number through overlap of two frequency signals in a time domain. That is, the number is represented by overlapping a signal with higher frequency and a signal with lower frequency, and such characteristic enables the numerical information sent through the dual-tone multifrequency to be mixed in voices and be differentiated and identified by a decoder.
Since numbers and character information such as * and # can be transferred in the voice call with the dual-tone multifrequency technology, the technology is widely applied to services and selection information transmission in the voice call. For example, after a subscriber dials customer service numbers of organizations such as operators and banks and so on, numbers 1, 2, 3 and 4, etc. input by the subscriber later are sent through the dual-tone multifrequency. At this point, a voice receiving server side of the operators and banks has a dual-tone multifrequency decoder, the decoder can identify the numbers 1, 2, 3 and 4, etc. input by the subscriber after converting the transmitted voices to the frequency domain, and perform a corresponding operation according to the identified number, such as operations of selecting type of service, entering card number or entering password and so on.
Even though the dual-tone multifrequency function gives an effective information transmission approach between mobile subscribers and service providers, few common subscribers perform information transmission with the function in the call. Therefore, a majority of mobile phones only integrate a dual-tone multifrequency coder in a voce call function at present, but does not integrate a dual-tone multifrequency decoder.
However, the mobile subscribers usually encounter a following difficulty in the process of actually using the mobile phones: in a call process between A and B, A tells B a telephone number, B does not have paper and pencil on hands at the moment and cannot record the number; or though B has the paper and pencil and records the telephone number, due to problems such as voice call environment noise and so on, it causes that the recorded number is inconsistent with the number told by A. Thus a trouble is brought to the two parties of the call.